a. Field of the Invention
Broadly speaking, this invention relates to the transmission of digital signals. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for differentially encoding the bit stream in a digital transmission system of the type which employs one or more parity bits to detect errors which occur during the transmission process.
B. Discussion of the Prior Art
As is well known, in recent years considerable attention has been focused on the establishment of an all-digital transmission network. Such a network would carry digital data signals as well as digitized (PCM) analog voice signals. The microwave system known in the industry as the 3A Radio Digital System (3A-RDS) is destined to play an important role in the establishment of such a network.
The 3A Radio Digital Terminal (3A-RDT), a key part of the 3A Radio Digital System, is designed to carry the Bell System's DS3 level digital signal via the existing TN-1 microwave radio system which operates in the 11 GHz common carrier band. The 3A-RDT is essentially a modem which serves as an interface between the digital, bipolar, DS3 signal and the TN-1 microwave radio system. In addition to its function as a modem, the 3A-RDT contains the performance monitor for the system. The Violation Monitor and Remover (VMR) within the digital receiver makes use of the parity bits included in the DS3 signal to evaluate the bit error rate and to determine the need for automatic switching to a standby protection channel.
As in most digital radio systems, the non-linearity of the radio transmitter dictates the use of angle modulation rather than amplitude modulation. The 3A-RDT transmitter uses 4-level, phase-shift-keying of a 70 MHz carrier in order to generate the IF signal needed as an input to the TN-1 transmitter. In the digital receiver, coherent demodulation is used to provide maximum immunity to the thermal noise of the TN-1 radio receiver. This noise is normally negligible in comparison to the received signal level, but heavy rain along the TN-1 route can cause severe fading of 11 GHz signals, resulting in a substantial reduction in the signal-to-noise ratio at the output of the radio receiver. Under such circumstances, the noise immunity of the digital receiver becomes a primary factor governing the reliability of the system.
The need for differential encoding of the digital signal in 3A-RDS is linked to the coherent demodulation employed in the digital terminal receiver. As is well known, coherent demodulation of a phase-modulated signal requires that the receiver recover an unmodulated carrier from the received signal which is then used as a phase reference. Various considerations in the design of the 3A-RDT dictated that carrier recovery be accomplished by phase-locking a voltage-controlled oscillator (VCO) operating at the IF frequency to a constant-phase signal at four times the IF frequency, this latter signal being generated by passing a portion of the four-phase received IF signal through a X4 multiplier. Despite the fact that after the phase-locked loop acquires "lock" the VCO provides a good, constant-phase reference signal, this approach does have one drawback. Because the phase of a signal at the IF frequency (the VCO output) is controlled by a tone of four times the IF frequency (the X4 multiplier output), there is a four-fold ambiguity in the resulting VCO phase, i.e., the VCO phase may come to rest at the desired phase (corresponding to the phase of the carrier used for modulation at the digital transmitter) or it may come to rest in a phase state differing from the desired phase by any integral multiple of 90.degree.. The consequence of this is that the receiver has no absolute phase reference, but has instead a reference against which only phase changes can be measured. Of course, this problem is not unique to 3A-RDS but is shared by many digital transmission systems using 4-phase modulation. Fortunately, it is well known that differential encoding of the digital signal can be used to ensure that the information is transmitted as phase changes, so that an absolute phase reference at the receiver is unnecessary.
Had the problem of recovered carrier phase ambiguity been the only constraint on the choice of a differential encoding scheme for 3A-RDS, no invention would have been needed. Known differential encoding schemes are more than adequate for encoding the digital information so that it is represented by phase changes rather than by any absolute quantity. However, the 3A-RDS system imposes an additional constraint on the choice of a suitable differential encoding scheme because of the requirement that the digital terminal receiver determine the bit error rate by monitoring the parity bits included in the DS3 bit stream. Obviously, the monitoring of the parity bits can only take place after differential decoding and, when conventional differential codes are used, it has been found that the decoding procedure alters the characteristics of the error occurrences in such a way that errors in the bit stream at the output of the decoder occur only in even numbers. This is a serious problem with 3A-RDS since the DS3 parity bits are implemented in such a way that even numbers of errors cannot be detected.
For a period of time, no differential code could be found which was compatible with the need to detect errors by monitoring the DS3 parity bits. During this period, the preliminary design of the 3A-RDT resolved the problem of phase ambiguity in the recovered carrier by using a "leaked" carrier approach rather than differential encoding. This approach consisted of transmitting a very small amount of unmodulated carrier signal along with the phase-modulated data signal. The carrier signal was so weak as to preclude its detection and use as a phase reference directly, but by first establishing a phase reference via the X4 multiplier method, the phase of the leaked signal could be sensed. This information was then used to correct for the possibly erroneous phase state of the phase reference generated by the X4 multiplier method. Unfortunately, the high degree of isolation needed between the weak, leaked carrier signal and various high-level signals in the receiver made this system impractical. Furthermore, the use of a higher level leaked carrier is not possible because the spurious tones generated by non-linearities in the TN-1 transmitter would probably violate FCC emission limitations.
Because the known differential codes were incompatible with the requirement that the bit error rate be determined by monitoring the DS3 parity bits, and because of the impracticality of the leaked carrier approach, the problem of the phase ambiguity in the recovered carrier threatened the viability of the 3A-RDT design. It was at this juncture that the new differential encoding scheme disclosed and claimed herein was invented for use with 3A-RDS and other similar systems suffering from the same problem.